Safety laws generally require that large machines such as earth-moving machines and vehicles be equipped with some form of automatic parking brake to prevent the vehicle from moving or rolling down an incline or the like when the machine is unattended or not in operation. Such machines are usually equipped with spring-applied brakes which are automatically applied when the vehicle is shut down or there is a loss of control pressure within the vehicle. These brake systems are also normally responsive to the control pressure of the vehicle to automatically release upon starting of the vehicle and the control pressure coming up to a predetermined level.
One problem with such systems is, however, that the parking brakes are normally for emergency use only and are generally not sufficiently powerful to prevent the vehicle engine from moving the vehicle. Thus, if the operator should fail to release the brake or if the control pressure of the vehicle should not attain a level sufficiently high to completely release the brakes, movement of the vehicle can then damage the brakes and render them substantially ineffective.